Borrowings - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 1,600,000,000
Outstanding short term letters of credit	382,900,000	$ 234,200,000
Amount of maintained federal fund lines of credit with correspondent banks	367,000,000	367,000,000
Investment securities collateral for Federal Reserve Bank Discount Window Lending program	66,200,000	52,000,000
Outstanding amount of federal fund line of credit with Federal Reserve Bank of Philadelphia	0	0
Amount of maintained line of credit with correspondent bank - parent company	10,000,000
Outstanding amount of line of credit with correspondent bank - parent company	0	$ 0
Securities Sold under Agreements to Repurchase [Member]
Debt Instrument [Line Items]
Long-term debt with fixed interest rate	5,000,000
Securities Sold under Agreements to Repurchase [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Long-term debt with variable interest rate	$ 25,400,000